This invention relates to polymeric additives providing increased plasticity, density and strength in the formation of simulated natural substances, such as stone-like and mineral-like materials. In particular the present invention involves the use of specific polymers together with mineral matrix materials, to provide green bodies of ceramic compositions, such as ceramic tiles and ceramic tiles with a granite-like or marble texture, having improved compaction, green density, green strength and surface finish properties.
Ceramic materials are often used to prepare lightweight, strong, thermally and chemically resistant products useful as chromatographic media, grinding aids, abrasives, catalysts, adsorbents, tableware, tiles, electronic components, construction components and machine components.
There currently exists a need for ceramic materials that mimic the appearance of mineral-like materials, such as natural stone, and in particular granite or marble, for use in the manufacture of flooring, tiles, counter tops, sinks, spas, sanitaryware, architectural articles and other ornamental materials.
In the manufacture of ceramic tiles, ceramic materials in the form of a powder are subjected to elevated pressures to produce what is known as a green body. One of the methods for compacting, or subjecting the materials to elevated pressures, to produce ceramic green bodies is pressing. Pressing methods include dry pressing, isostatic pressing and semi-wet pressing. Using these methods, the green bodies can be prepared in various shapes and sizes. In dry pressing ceramic tiles, ceramic materials are first processed into a slurry, converted into granules and then compacted into green bodies having various shapes and sizes. In semi-wet pressing, ceramic materials are processed into a wet mix and compacted into green bodies having various shapes and sizes.
The properties of the green bodies generally affect the properties of the final ceramic tiles. The final product is typically prepared by sintering the green body. If the green density of the green body is too low, the mechanical properties of the final tiles, such as hardness and toughness, will diminish. If the green strength is too low, it becomes difficult or impossible to process the green body. Thus, it is desirable to provide ceramic green bodies with sufficient green densities and green strengths.
Green density is determined by how well the ceramic materials are compacted during processing. For example, in dry pressing, the green density is determined by the compaction of the granules. Compaction of granules can be improved by introducing plasticity into the granule composition.
Lack of plasticity in ceramic granules results in increased hardness of the granules. Increased hardness reduces compaction of the granules, and therefore reduces density of the green bodies after pressing; low green density results in a low fired density after sintering which reduces mechanical strength of the final ceramic product. Increased hardness of granules can also cause a rough surface finish in the pressed green bodies, introducing potential defects into the final ceramic product after sintering.
Plasticity becomes even more important in increasing green density and improving surface finish when the granule size is large; for example, large size granules are preferred in order to achieve a desired granite-like or marble texture in certain finished ceramic materials. With larger size granules, increased plasticity is required to compact the large granules into dense green bodies. This plasticity is crucial in pressing tiles with granite-like or marble texture in which the granules can be as large as a few millimeters (mm).
Green strength is equally important in the green bodies of ceramic tiles as it provides the necessary handling characteristics of the green bodies. Green strength can also be affected by the compaction of ceramic materials in the green bodies. Higher compaction results in higher green strength, other factors being equal.
One method for increasing compaction, thus increasing green density and green strength of ceramic green bodies, is to use a plasticizer as a processing aid in the preparation of the green bodies. Common plasticizers include water, ethylene glycols, polyethylene glycols, glycerol, dibutyl phthalate and dimethyl phthalate (James S. Reed, Principles of Ceramic Processing, Second Ed., John Wiley and Sons, p 204 New York, 1995). Those plasticizers are either water-soluble or water-insoluble. The water-insoluble plasticizers are not suitable for pressing of ceramic tiles since the plasticizers cannot be incorporated into a ceramic slurry that is water based. The water-soluble plasticizers are also less desirable since they are sensitive to changes in humidity, that is, they are hygroscopic. Thus, ceramic green bodies made by processes using these water-soluble plasticizers exhibit variability in compaction, green density, green strength, shrinkage and die sticking as the humidity varies (Whitman et al., "Humidity Sensitivity of Dry Press Binders," Paper No. SXVIIb-92-94, presented at the 96.sup.th Annual Meeting of the American Ceramic Society, Indianapolis, Ind., Apr. 25, 1994). In addition, since the aforementioned plasticizers are either small molecules or very low molecular weight polymers, they provide little or no adhesion for ceramic powders during processing. The ceramic green bodies processed with these plasticizers have little strength.
U.S. Pat. No. 4,968,460 discloses several classes of emulsion polymers useful as binders for ceramic materials, including acrylate ester polymers, having glass transition temperatures from -100.degree. C. to +120.degree. C. These binders provide increased green strength and green density to the ceramic green bodies when the green body is subjected to an energy treatment step, such as electron beam irradiation, X-ray irradiation, ultraviolet radiation, heating treating from 50.degree. C. to 200.degree. C. or combinations of heat and pressure; the additional treatments add time and cost during production of ceramic green bodies.
The present invention seeks to overcome the problems involving the use of prior art additives in preparing ceramic compositions by using selected polymer additives having high molecular weight, low T.sub.g and low-hygroscopicity: the low T.sub.g ensures sufficient plasticization of the mineral matrix material and the high molecular weight/low-hygroscopicity properties provide sufficient green strength.